Membrane, biological cell dimensions

It is obvious that the combination of nanoscience/nanotechnology with medicine makes a lot of sense for many reasons. One major reason is that the nanoscale is a characteristic biological scale, a scale related directly to life. DNA strains, globular proteins such as ferricins, vimses are all on this scale. For instance, the tobacco mosaic vims is actually a nanotube. The dimensions of bacteria and of cells tend to be already in the micrometer range, but important subunits of these objects such as the membranes of cells have dimensions in the nm scale. 

In a series of experiments, we produced giant vesicles with dimension of several microns up to several millimeters by phase transfer processes [1,2]. Giant vesicles are often used as model membranes for biological cells. Due to their size, it is possible to investigate typical effects like budding, fission or membrane fusion by... 

Compounds can cross biological membranes by two passive processes, transcellu-lar and paracellular mechanisms. For transcellular diffusion two potential mechanisms exist. The compound can distribute into the lipid core of the membrane and diffuse within the membrane to the basolateral side. Alternatively, the solute may diffuse across the apical cell membrane and enter the cytoplasm before exiting across the basolateral membrane. Because both processes involve diffusion through the lipid core of the membrane the physicochemistry of the compound is important. Paracellular absorption involves the passage of the compound through the aqueous-filled pores. Clearly in principle many compounds can be absorbed by this route but the process is invariably slower than the transcellular route (surface area of pores versus surface area of the membrane) and is very dependent on molecular size due to the finite dimensions of the aqueous pores. 

If f<total potential difference applied across the cell is developed across the membrane capacitance. In this limit, the induced membrane potential AV across a spherical cell is AV = 1.5 ER, where E represents the applied external field. Thus the cell samples the external field strength over its dimensions and delivers this integrated voltage to the membranes, which is a few mV at these low frequencies for cells larger than 10 ym and external fields of about 1 V/cm. These transmembrane potentials can be biologically significant. 

The large dimensions necessary for biological responses to weak microwave fields might be achieved by a cooperative reaction of a number of cells or macromolecules to the microwave stimulus, which increases the effective size of the structure and correspondingly reduces the threshold that is required for an effect. Adey suggested that such cooperativity might be induced in the counterions loosely bound near membrane surfaces which contain a loose frame work of charged polysaccharides (32). 

An EFC consists of two electrodes, anode and cathode, connected by an external load (shown schematically in Figure 5.1). In place of traditional nonselective metal catalysts, such as platinum, biological catalysts (enzymes) are used for fuel oxidation at the anode and oxidant reduction at the cathode. J udicious choice of enzymes allows such reactions to occur under relatively mild conditions (neutral pH, ambient temperature) compared to conventional fuel cells. In addition, the specificity of the enzyme reactions at the anode and cathode can eliminate the need for other components required for conventional fuel cells, such as a case and membrane. Due to the exclusion of such components, enzymatic fuel cells have the capacity to be miniaturized, and consequently micrometer-dimension membraneless EFCs have been developed [7]. In the simplest form, the difference between the formal redox potential (F ) of the active site of the enzymes utilized for the anode and cathode determines the maximum voltage (A ) of the EFC. Ideally enzymes should possess the following qualities. 

Here three template systems are discussed one that is achieved by assembling monodisperse spheres into a colloidal crystal, the second a membrane (polycarbonate) consisting of cylindrical pores of set dimensions, and the third a biological structure composed of ordered spherical yeast cells. These molds have been cast in most examples, unless blockage of smaller pores prevented the complete filling of larger pores. 

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